Communications networks can suffer from failures or service degradation, both of which often cause tangible losses to users as well as the network providers. Redundant transmission on physically diverse paths is a common precaution against the effects of failures in communication networks. Hereinafter, physically diverse is defined as meaning that a single failure will affect only one given path. In optical fiber networks the restoration time must be as short as possible since line bit rates are very high. Furthermore, some services require a high degree of reliability because they may be adversely affected by any amount of bit loss. For these types of services, protection switching without bit loss (hitless protection switching, hereafter) is desired.
Typically, in a communication network a very small portion of the traffic requires highly reliable service, such as voice and video traffic. In some cases, for example with 911 services, the traffic is fault intolerant in that it cannot afford to have call set up failure or to be dropped, and hence requires hitless switching. In a mesh network, extremely fast restoration would be required to prevent such calls from being dropped in the case of a fault. In other cases, a customer may be willing to pay for a higher degree of reliability (e.g. for mission critical applications) and in these cases it may also be desirable to provide hitless protection switching.
Known approaches that provide hitless switching usually involve error detection, synchronization, and selection algorithms. For example, in a paper entitled “A New Synchronization Algorithm for Hitless Protection Switching in ATM Networks” in IEEE paper 0-7803-525-0/99 by Andreas Iselt, a method of hitless protection switching is described. Synchronization of the two data streams is an important aspect of the method and involves three phases: a hunt phase, a validation phase and a monitoring phase. While the method described by Iselt provides adequate synchronization for hitless switching, it may be quite complex to implement. Furthermore, it is not easily adapted to provide greater protection, for example using more than two paths, which could be useful for 911 services for example, to protect against multiple faults in the network.
Therefore, a method of protecting traffic that is simple to implement and is scalable for selectable degrees of reliability against network faults is desired.